Spiro[5.5]undeca-1,4,8-trien-3-ones are known compounds that are described in Hatchard, Journal of the American Chemical Society, Vol. 80, pp. 3640-3642 (1958); McClure, Journal of Organic Chemistry, Vol. 27, pp. 2365-2368 (1962); and copending application Ser. No. 472,196, filed Mar. 4, 1983, in the name of Jerry M. Roper (Roper). As taught in Roper, such spiroketones are of interest as antioxidants and/or as flame retardant, insecticide, or pharmaceutical intermediates; and they can be prepared by reacting a 4-aminomethylphenol with a conjugated diene and an alkyl halide, typically at a temperature in the range of about 50.degree.-200.degree. C., in an inert solvent. Roper also teaches that (1) his reaction proceeds via alkylation of the phenol reactant to a quaternary ammonium salt which subsequently forms a quinone methide that then undergoes cycloaddition with the diene and (2) the use of at least one molar equivalent of the alkyl halide is required in his process.